How to add a custom initializer to a struct without losing its memberwise initializer

Swift version: 5.6

Paul Hudson    @twostraws    May 28th 2019

All structs in Swift come with a default memberwise initializer, which is an initializer that accepts values for all the properties in the struct. However, as soon as you add your own initializer to the struct that memberwise initializer goes away, because it’s possible you’re doing special work that the default initializer isn’t aware of.

If you want to keep both the default initializer and your own custom ones, there’s a simple trick: create your initializers inside an extension rather than as part of the main struct definition.

For example:

struct Person {
    var firstName: String
    var lastName: String
}

extension Person {
    init(name: String) {
        let split = name.components(separatedBy: " ")
        firstName = split.first ?? ""
        lastName = split.last ?? ""
    }
}

Because my custom initializer is inside an extension, you can create instances of Person in two ways:

let taylor1 = Person(firstName: "Taylor", lastName: "Swift")
let taylor2 = Person(name: "Taylor Swift")

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